Yard MD blog: Japanese knotweed is an elegant invader

Quickly rising up to 10 feet tall in spring, the imposing, arched stems of Japanese knotweed quickly smother native plants when it moves into a new area.
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As I’ve stressed over and over here in my series of articles during Invasive Plant Awareness Month, it is often the common name used by various gardeners, growers and organizations that creates a great deal of confusion when it comes to regulating and understanding invasive plants.

The plant I am featuring today is one of the best examples of this. Depending upon who you speak to, or where you look online, this plant goes by many different names, including Mexican bamboo, Chinese bamboo, Japanese knotweed, fleeceflower and many more. The latin, or scientific name is Polygonum cuspidatum, but that is of little to no value when the plant is not for sale, but simply on a rampage through our native habitats.

I’ll go with Japanese knotweed, since that appears to be the common name used most often. A related species, also invasive, called giant knotweed, is nearly identical in habit, form and appearance.

Whatever you call it, Japanese knotweed is one of the most imposing of all invasive plants, growing quickly to 10 feet or tall, with thick, bamboo-like, hollow stems and dense, intertwined roots and rhizomes that are nearly impossible to eradicate once the plant becomes established.

The name fleeceflower is familiar to many gardeners for the many garden varieties that are currently available. Fleeceflowers bloom at the tip of the stalk in a mass of fluffy, white blooms. Japanese knotweed is a member of the garden fleeceflower family. The flowers are borne all along the stalk, though most heavily concentrated near the tip. It blooms from midsummer through early fall.

The tall, gracefully arched stalks of this giant plant are deceptively attractive, with reddish green stems and large, heart-shaped leaves that are evenly spaced along the entire length of the plant.

Because it quickly forms immense stands of arching growth that can reach maximum height in just a few short weeks in late spring, Japanese knotweed is a dangerous invader that quickly overtakes large areas and spreads through underground rhizomes.

Plants are easy enough to cut or pull, but repeated action is needed, often for years, once the plant is established, because the underground and surface rhizomes spread so voraciously. Any small piece left in the ground will multiply and spread. Unfortunately, cutting only stimulates the rhizome below, meaning once you begin cutting, you will need to continue for as long as it takes to finally eradicate the plant. This could mean several seasons.

Herbicides such as Roundup will work, but only on the fresh cut cane rim (the edge of the hollow) and only if applied literally within minutes of a fresh cut. If you wait much longer than that, the plant has already sealed itself back up and the Roundup will not be effective. Apply only to the cut stems, preferably with a brush. Repeated applications and repeated cuttings will be needed, again, often over multiple growing seasons.

Colonies of Japanese knotweed, like so many other invasives, quickly shade out native plants on the ground, taking over roadsides, hillsides, parks, even backyard gardens.